There is a recent trend to use a double suction vertical pump, instead of a single suction vertical pump, as a pump installed in a suction pit. The double suction vertical pump has an advantage of improved suction performance because a flow rate of water into each suction opening is approximately half of that in the single suction vertical pump. Therefore, NPSH (Net Positive Suction Head) can be low. The improved suction performance enables the pump to perform its pumping operation at a low water level, and thus the suction pit can be made shallow. Therefore, cost reduction of the suction pit can be achieved.
Further, the improved suction performance can make cavitation less likely to occur at an impeller inlet and can reduce adverse influences of the cavitation (e.g., bubbling, cavitation damage to impeller surface and casing surface resulting from collapse of bubbles). Thus, a set rotational speed of the impeller can be increased. As a result, the impeller can have a smaller diameter while maintaining its pumping performance, and pump size can be compact and cost reduction of the pump itself can be achieved.
Although the double suction vertical pump has the advantage of preventing the cavitation because of the half flow rate of water into each suction opening as compared with the single suction vertical pump, this type of pump is likely to cause an air entrained vortex developing from a water surface because one of two suction openings faces upward. This arrangement makes it difficult to lower the water level and to reduce the depth of the suction pit.